Soil aggregation and fungal and bacterial biomass under annual and perennial cropping systems

Understanding the mechanisms controlling crop effects on soil aggregation is necessary to develop sustainable soil management practices. Changes in soil aggregation, carbohydrates, and fungal vs. bacterial biomass were monitored following a shift from fallow (bare soil) to various cropping systems. Faba bean (Vicia faba L.) and wheat (Triticum aeslivum L.) were used as annual cropping systems, while alfalfa (Medicago sativa L.), timothy (Phleum pratense L.), bromegrass (Bromus inermis L.), and reed canarygrass (Phalaris arundinacea L.) were used as perennial cropping systems. Crops were established in spring 1989, on a silty clay loam (fine, mixed, frigid Typic Dystro-chrept) and a clay loam (fine-loamy, mixed, frigid Typic Humaquept) near Quebec City (Canada). After three growing seasons, the mean-weight diameter of water-stable aggregates (MWD) was higher under reed canarygrass (2.26 mm in the clay loam and 2.45 mm in the silty clay loam) and timothy (2.13 and 2.26 mm) than under faba bean (1.92 and 1.89 mm) or wheat (2.06 and 1.57 mm). Intermediate values were found under alfalfa and bromegrass. Changes in MWD were not correlated with microbial biomass C. Acid-hydrolyzable carbohydrates correlated with MWD in the silty clay loam (r(2) = 0.42**) but the relationship decreased with higher carbohydrate levels in the clay loam (r(2) = 0.05), Close correlations were found between MWD and both fungal glucosamine (r(2) = 0.68***, soils combined) and bacterial muramic acid (r(2) = 0.48***). Changes in MWD were mostly reflected in aggregates >2 mm, and the close relationship with fungal glucosamine suggests that fungi played a dominant role in soil macroaggregation. Measurement of muramic acid and glucosamine is proposed as a potential approach to compare bacterial vs. fungal contributions to soil aggregation.